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The outer layers are also known as the external copper layers of the PCB. Normally, the electronic components are attached to the outer layers by soldering them into through holes or onto surface mount pads.

Outer layers generally begin with a copper foil that is then electroplated with copper to increase the thickness and add copper to the through-hole barrels. The copper weight of the outer layers is predefined for the Standard Constructions. The finished copper thickness of these layers, however, can be customized to meet your needs by selecting non standard construction in the ordering process and uploading a stackup in your file set. 

Minimum manufacturable trace widths and spacing requirements are based on the pre-plate (foil) copper weight of the layer.